1,507 research outputs found
The second US Naval Observatory CCD Astrograph Catalog (UCAC2)
The second USNO CCD Astrograph Catalog, UCAC2 was released in July 2003.
Positions and proper motions for 48,330,571 sources (mostly stars) are
available on 3 CDs, supplemented with 2MASS photometry for 99.5% of the
sources. The catalog covers the sky area from -90 to +40 degrees declination,
going up to +52 in some areas; this completely supersedes the UCAC1 released in
2001. Current epoch positions are obtained from observations with the USNO
8-inch Twin Astrograph equipped with a 4k CCD camera. The precision of the
positions are 15 to 70 mas, depending on magnitude, with estimated systematic
errors of 10 mas or below. Proper motions are derived by utilizing over 140
ground-and space-based catalogs, including Hipparcos/Tycho, the AC2000.2, as
well as yet unpublished re-measures of the AGK2 plates and scans from the NPM
and SPM plates. Proper motion errors are about 1 to 3 mas/yr for stars to 12th
magnitude, and about 4 to 7 mas/yr for fainter stars to 16th magnitude. The
observational data, astrometric reductions, results, and important information
for the users of this catalog are presented.Comment: accepted by AJ, AAS LaTeX, 14 figures, 10 table
The StarScan plate measuring machine: overview and calibrations
The StarScan machine at the U.S. Naval Observatory (USNO) completed measuring
photographic astrograph plates to allow determination of proper motions for the
USNO CCD Astrograph Catalog (UCAC) program. All applicable 1940 AGK2 plates,
about 2200 Hamburg Zone Astrograph plates, 900 Black Birch (USNO Twin
Astrograph) plates, and 300 Lick Astrograph plates have been measured. StarScan
comprises of a CCD camera, telecentric lens, air-bearing granite table, stepper
motor screws, and Heidenhain scales to operate in a step-stare mode. The
repeatability of StarScan measures is about 0.2 micrometer. The CCD mapping as
well as the global table coordinate system has been calibrated using a special
dot calibration plate and the overall accuracy of StarScan x,y data is derived
to be 0.5 micrometer. Application to real photographic plate data shows that
position information of at least 0.65 micrometer accuracy can be extracted from
course grain 103a-type emulsion astrometric plates. Transformations between
"direct" and "reverse" measures of fine grain emulsion plate measures are
obtained on the 0.3 micrometer level per well exposed stellar image and
coordinate, which is at the limit of the StarScan machine.Comment: 24 pages, 8 figures, accepted for PAS
Deep Astrometric Standards (DAS) and Galactic Structure
The advent of next-generation imaging telescopes such as LSST and Pan-STARRS
has revitalized the need for deep and precise reference frames. The proposed
weak-lensing observations with these facilities put the highest demands on
image quality over wide angles on the sky. It is particularly difficult to
achieve a sub-arcsecond PSF on stacked images, where precise astrometry plays a
key role. Current astrometric standards are insufficient to achieve the science
goals of these facilities. We thus propose the establishing of a few selected
deep (V=25) astrometric standards (DAS). These will enable a reliable geometric
calibration of solid-state mosaic detectors in the focal plane of large
ground-based telescopes and make a substantial contribution to our
understanding of stellar populations in the Milky Way. In this paper we
motivate the need for such standards and discuss the strategy of their
selection and acquisition and reduction techniques. The feasibility of DAS is
demonstrated by a pilot study around the open cluster NGC 188, using the KPNO
4m CCD Mosaic camera, and by Subaru Suprime-Cam observations. The goal of
reaching an accuracy of 5-10 mas in positions and obtaining absolute proper
motions good to 2 mas/yr over a several square-degree area is challenging, but
reachable with the NOAO 4m telescopes and CCD mosaic imagers or a similar
set-up. Our proposed DAS aims to establish four fields near the Galactic plane,
at widely separated coordinates. In addition to their utilitarian purpose for
DAS, the data we will obtain in these fields will enable fundamental Galactic
science in their own right. The positions, proper motions, and VI photometry of
faint stars will address outstanding questions of Galactic disk formation and
evolution, stellar buildup and mass assembly via merger events.Comment: 41 pages, 9 figures, accepted for publication in PAS
Application of mineralogical, petrological and geochemical tools for evaluating the palaeohdrogeological evolution of the PADAMOT study sites
The role of Work Package (WP) 2 of the PADAMOT project â âPalaeohydrogeological Data
Measurementsâ - has been to study late-stage fracture mineral and water samples from
groundwater systems in Spain, Sweden, United Kingdom and the Czech Republic, with the aim
of understanding the recent palaeohydrogeological evolution of these groundwater systems. In
particular, the project sought to develop and evaluate methods for obtaining information about
past groundwater evolution during the Quaternary (about the last 2 million years) by examining
how the late-stage mineralization might record mineralogical, petrographical and geochemical
evidence of how the groundwater system may have responded to past geological and
climatological changes.
Fracture-flow groundwater systems at six European sites were studied:
âą Melechov Hill, in the Bohemian Massif of the Czech Republic: a shallow (0-100 m)
dilute groundwater flow system within the near-surface weathering zone in fractured
granitic rocks;
âą Cloud Hill, in the English Midlands: a (~100 m) shallow dilute groundwater flow system
in fractured and dolomitized Carboniferous limestone;
âą Los Ratones, in southwest Spain: an intermediate depth (0-500 m) dilute groundwater
flow system in fractured granitic rocks;
âą Laxemar, in southeast Sweden: a deep (0-1000 m) groundwater flow system in fractured
granitic rocks. This is a complex groundwater system with potential recharge and
flushing by glacial, marine, lacustrine and freshwater during the Quaternary;
âą Sellafield, northwest England: a deep (0-2000 m) groundwater flow system in fractured
Ordovician low-grade metamorphosed volcaniclastic rocks and discontinuous
Carboniferous Limestone, overlain by a Permo-Triassic sedimentary sequence with
fracture and matrix porosity. This is a complex coastal groundwater system with deep
hypersaline sedimentary basinal brines, and deep saline groundwaters in crystalline
basement rocks, overlain by a shallow freshwater aquifer system. The site was glaciated
several times during the Quaternary and may have been affected by recharge from glacial
meltwater;
âą Dounreay, northeast Scotland: a deep (0-1400 m) groundwater flow system in fractured
Precambrian crystalline basement overlain by fractured Devonian sedimentary rocks.
This is within the coastal discharge area of a complex groundwater system, comprising
deep saline groundwater hosted in crystalline basement, overlain by a fracture-controlled
freshwater sedimentary aquifer system. Like Sellafield, this area experienced glaciation
and may potentially record the impact of glacial meltwater recharge.
In addition, a study has been made of two Quaternary sedimentary sequences in Andalusia in
southeastern Spain to provide a basis of estimating the palaeoclimatic history of the region that
could be used in any reconstruction of the palaeoclimatic history at the Los Ratones site:
âą The CĂșllar-Baza lacustrine sequence records information about precipitation and
palaeotemperature regimes, derived largely from the analysis of the stable isotope (ÎŽ18O
and ÎŽ13C) signatures from biogenic calcite (ostracod shells).
âą The Padul Peat Bog sequence provided information on past vegetation cover and
palaeogroundwater inputs based on the study of fossil pollen and biomarkers as proxies
for past climate change.
Following on from the earlier EC 4th Framework EQUIP project, the focus of the PADAMOT
studies has been on calcite mineralization. Calcite has been identified as a late stage mineral, closely associated with hydraulically-conductive fractures in the present-day groundwater
systems at the Ăspö-Laxemar, Sellafield, Dounreay and Cloud Hill sites. At Los Ratones and
Melechov sites late-stage mineralization is either absent or extremely scarce, and both the
quantity and fine crystal size of any late-stage fracture mineralization relevant to Quaternary
palaeohydrogeological investigations is difficult to work with. The results from the material
investigated during the PADAMOT studies indicate that the fracture fillings at these sites are
related to hydrothermal activity, and so do not have direct relevance as Quaternary indicators.
Neoformed calcite has not been found at these two sites at the present depth of the investigations.
Furthermore, the HCO3
- concentration in all the Los Ratones groundwaters is mainly controlled
by complex carbonate dissolution. The carbonate mineral saturation indices do not indicate
precipitation conditions, and this is consistent with the fact that neoformed calcite, ankerite or
dolomite have not been observed petrographically
Hyperfine Spectroscopy of Optically Trapped Atoms
We perform spectroscopy on the hyperfine splitting of Rb atoms trapped
in far-off-resonance optical traps. The existence of a spatially dependent
shift in the energy levels is shown to induce an inherent dephasing effect,
which causes a broadening of the spectroscopic line and hence an inhomogeneous
loss of atomic coherence at a much faster rate than the homogeneous one caused
by spontaneous photon scattering. We present here a number of approaches for
reducing this inhomogeneous broadening, based on trap geometry, additional
laser fields, and novel microwave pulse sequences. We then show how hyperfine
spectroscopy can be used to study quantum dynamics of optically trapped atoms.Comment: Review/Tutoria
Cartan subalgebras and the UCT problem, II
We show that outer approximately represenbtable actions of a finite cyclic
group on UCT Kirchberg algebras satisfy a certain quasi-freeness type property
if the corresponding crossed products satisfy the UCT and absorb a suitable UHF
algebra tensorially. More concretely, we prove that for such an action there
exists an inverse semigroup of homogeneous partial isometries that generates
the ambient C*-algebra and whose idempotent semilattice generates a Cartan
subalgebra. We prove a similar result for actions of finite cyclic groups with
the Rokhlin property on UCT Kirchberg algebras absorbing a suitable UHF
algebra. These results rely on a new construction of Cartan subalgebras in
certain inductive limits of Cartan pairs. We also provide a characterisation of
the UCT problem in terms of finite order automorphisms, Cartan subalgebras and
inverse semigroups of partial isometries of the Cuntz algebra .
This generalizes earlier work of the authors.Comment: minor revisions; final version, accepted for publication in Math.
Ann.; 26 page
Basal forebrain cholinergic system volume is associated with general cognitive ability in the elderly
OBJECTIVE: At the present, it is unclear whether association of basal forebrain cholinergic system (BFCS) volume with cognitive performance exists in healthy as well as in cognitively impaired elderly subjects. Whereas one small study reported an association of BFCS volume with general cognitive ability 'g' in healthy ageing, effects on specific cognitive domains have only been found in subjects with cognitive decline. Here we aim to clarify whether an association of BFCS volume and 'g' is present in a larger sample of elderly subjects without obvious symptoms of dementia and whether similar associations can also be observed in specific cognitive domains. METHODS: 282 pre-surgical patients from the BioCog study (aged 72.7±4.9 years with a range of 65-87 years, 110 women) with a median MMSE score of 29 points (range 24-30) were investigated. BFCS and brain volume as well as brain parenchymal fraction were assessed in T1-weighted MR images using SPM12 and a probabilistic map of the BFCS. Neuropsychological assessment comprised the CANTAB cognitive battery and paper-and-pencil based tests. For data analysis, generalised linear models and quantile regression were applied. RESULTS: Significant associations of BFCS volume with 'g' and several cognitive domains were found, with the strongest association found for 'g'. BFCS volume explained less variance in cognitive performance than brain volume. The association was not confounded by brain parenchymal fraction. Furthermore, the association of BFCS volume and 'g' was similar in high- and low-performers. CONCLUSION: Our results extend previous study findings on BFCS volume associations with cognition in elderly subjects. Despite the observed associations of BFCS volume and cognitive performance, this association seems to reflect a more general association of brain volume and cognition. Accordingly, a specific association of BFCS volume and cognition in non-demented elderly subjects is questionable
AN ULTRA-FAINT GALAXY CANDIDATE DISCOVERED in EARLY DATA from the MAGELLANIC SATELLITES SURVEY
We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644-5953 (Pictor II or Pic II) is a low surface brightness (Ό = 28.5+1 -1 mag arcsec-2 within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of 45+5 -4 kpc. The physical size (r1/2 = 46+15 -11) and low luminosity (Mv = -3.2+0.4 -0.5 mag) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644-5953 (Pic II) is located 11.3+3.1 -0.9 kpc from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644-5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.Peer reviewe
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